1. Field of The Invention
The present invention relates to novel 6-position highly acetylated cellulose diacetates, and processes for the production of the 6-position highly acetylated cellulose diacetates. The novel 6-position highly acetylated cellulose diacetates are useful typically as materials for cellulose acylates of different acyl groups, which cellulose acylates have a high total degree of substitution, and are usable typically as materials for agents for separating optical isomers, materials for films typified by photographic materials and optical materials.
2. Description of The Related Art
Cellulose esters have superior optical properties and are thereby used typically as a variety of optical films. Polarizing plates, for example, are used in liquid crystal displays for use in flat-panel displays typified by liquid crystal display televisions. The polarizing plates are each generally composed of a laminate of a polarizing film with a polarizing plate-protecting film (polarizing film-protecting film) made of a cellulose ester film. Such optical films should have various optical properties, such as optical transparency and refractive index, according to the driving system of the liquid crystal devices. By way of example, refractive indices in in-plane thickness directions, respectively, should be controlled, and a birefringence in the in-plane direction should be increased in vertical alignment (VA) liquid crystal devices. An exemplary procedure for controlling these parameters is film drawing. However, among cellulose esters, a cellulose acetate has low drawability in itself, and control of refractive indices in the in-plane and thickness directions of the resulting cellulose acetate film is limited. In contrast, when a substituent having carbon atoms larger than that of acetyl group is introduced into an original cellulose acetate, the resulting cellulose acetate can have higher drawability. For example, film drawing at a high draw ratio is possible in cellulose esters of different fatty acids, such as a cellulose acetate propionate into which propionyl group having carbon atoms larger than that of acetyl group is introduced. Among such cellulose esters of different fatty acids, acetyl-containing cellulose esters of different fatty acids are more desirable. The possibility of film drawing at a high draw ratio enlarges the controllable area of refractive index, and this in turn enlarges the applicability of the resulting film as an optical film. Specifically, cellulose esters of different fatty acids are more advantageously used than cellulose acetates in uses where drawing is conducted.
There have been known cellulose acylates of different acyl groups, in which hydroxyl groups of a cellulose are substituted by two or more different acyl groups (e.g., acetyl group and another acyl group than acetyl group). Typically, Japanese Patent Publication (JP-A) No. 2002-322201 discloses a cellulose ester of different acids in which hydrogen atoms of hydroxyl groups of a cellulose are substituted by both a substituted or unsubstituted aromatic acyl group and a substituted or unsubstituted aliphatic acyl group. The document mentions that this cellulose ester of different acids can give a film superior in optical isotropy, optical transparency, water resistance, and dimensional stability. In the examples of this document, cellulose benzoate trifluoroacetate, for example, is synthetically prepared.
JP-A No. 2006-328298 discloses an optical film which is formed by melting a composition mainly containing a cellulose ester, in which the cellulose ester satisfies following Conditions (1) and (2):2.4≦X+Y≦2.9  (1)0.3≦Y≦1.5  (2)wherein X represents the degree of substitution by acetic acid; and Y represents the degree of substitution by an aromatic carboxylic acid.
In the examples of this document, cellulose acylates of different acyl groups, such as cellulose acetate benzoate, are synthetically prepared by reacting a cellulose as a material with two different carboxylic acids. This reaction, however, is a reaction under nonuniform conditions and does not proceed uniformly. It is considered that the reaction of a carboxylic acid having higher reactivity first occurs on the cellulose surface, and thereafter the reaction of another carboxylic acid having lower activity occurs. In addition, the product cellulose ester derivative is a composition of derivative molecules having a large molecule-to-molecule variation and includes, for example, an acetyl-rich derivative and a benzoyl-rich derivative. This composition has disadvantages such that constitutional derivatives have different solubilities in a solvent and thereby cause phase separation; if the composition is formulated into a dope, the dope becomes turbid, is difficult to filtrate or is impossible to filtrate; and the composition shows a large intermolecular distribution of degree of substitution.
JP-A No. 2007-199392 and JP-A No. 2007-199391 disclose cellulose acylate films having specific optical properties. The techniques disclosed in these documents, however, merely give cellulose acetate derivatives having a high degree of substitution by benzoyl group at the 6-position of a glucose backbone, and they fail to provide a cellulose acetate derivative having benzoyl group selectively introduced at the 2- and 3-positions of the glucose backbone.
As has been described above, there have been known cellulose acylates of different acyl groups in which hydroxyl groups of cellulose are substituted by acetyl group and other acyl groups than acetyl group; however, there is known no cellulose acylate of different acyl groups which has a very high degree of acetyl substitution at the 6-position of glucose backbone, a high degree of substitution with other acyl groups than acetyl group at the 2- and 3-positions of glucose backbone, and a high total degree of acyl substitution. The cellulose acylate of different acyl groups having such a distribution of substituents is expected to exhibit unprecedented specific optical properties.
Possible processes for the production of such a cellulose acylate of different acyl groups having the above-mentioned distribution of substituents include a process of introducing other acyl groups than acetyl group into free hydroxyl groups of a material cellulose acetate; a process of introducing acetyl group preferentially into hydroxyl group at the 6-position of a material cellulose acetate and subsequently introducing other acyl groups than acetyl group into the 2- and 3-positions of the cellulose acetate. In these processes, however, how acetyl group distributes as a substituent in the glucose ring of the material cellulose acetate is very important. Specifically, when hydroxyl group at a certain position of the glucose backbone has been substituted by acetyl group, it is difficult to introduce another acyl group into this position. Accordingly, when the above processes are employed, the starting material should be a cellulose acetate which has a controlled distribution of acetyl group in the glucose ring. In particular, a cellulose acetate having a high degree of acetyl substitution at the 6-position may be advantageous as the starting material.
Cellulose acetates are classified into cellulose triacetates, cellulose diacetates, and cellulose monoacetates, of which cellulose triacetates and cellulose diacetates are industrially important. In general, a cellulose acetate having a degree of acetyl substitution of 2.6 or more is designated as a “cellulose triacetate”, and one having a degree of acetyl substitution of 2 or more and less than 2.6 is designated as a “cellulose diacetate”. Cellulose acetates have largely varying properties depending typically on the degree of acetyl substitution and the degree of polymerization, and, by controlling these parameters, they can be used in a variety of applications.
Known cellulose acetates having a high degree of acetyl substitution at the 6-position are those having a high total degree of acetyl substitution, as mentioned below. However, there has been known no cellulose acetate that has a high degree of acetyl substitution at the 6-position, has a not-so-high total degree of acetyl substitution as to give margin of introducing other acyl groups than acetyl group to some extent, and has a relatively high molecular weight.
By way of example, JP-A No. 2005-97621 describes that, when a cellulose acetate is prepared according to a common procedure, the degree of acetyl substitution at the 2-position or 3-position is higher than the degree of acetyl substitution at the 6-position. This document discloses a technique of improving a cellulose acylate so as to have a low retardation value of the product film; and discloses a cellulose acetate that has a total degree of acetyl substitution at the 2-, 3-, and 6-positions of 2.67 or more, a total degree of acetyl substitution at the 2- and 3-positions of 1.97 or less, and a degree of acetyl substitution at the 6-position of 0.85 or more and 0.98 or less. JP-A No. 2005-68438 discloses a technique of improving a cellulose acylate so as to prepare a cellulose acylate solution that is superior in long-term stability and shows a low viscosity in practicable concentrations as a dope. There is disclosed a cellulose acylate that is controlled to have a total degree of acyl substitution at the 2- and 3-positions of 1.70 or more and 1.90 or less and a degree of acyl substitution at the 6-position of 0.88 or more. These cellulose acylates, however, each have a high total degree of acyl substitution and thereby have little margin of introducing other acyl groups than acetyl group into the 2- and 3-positions. These cellulose acylates are each prepared under such reaction conditions that the amount of catalytic sulfuric acid is reduced to prolong the acetylation reaction time. However, because of such a small amount of the catalytic sulfuric acid, the product cellulose acylates prepared under these reaction conditions show a large distribution in degree of substitution, include large amounts of insoluble components, and, if formed into an optical film, include large amounts of optical foreign matter. In addition, the product cellulose acylates are liable to have lower molecular weights, because the small amount of the catalytic sulfuric acid causes a lowered hydrolysis rate and thereby a prolonged hydrolysis process time in acylation and hydrolysis steps of the cellulose acylates.
JP-A No. 2002-338601 and JP-A No. 2003-201301 disclose techniques of preparing cellulose acetates by reacting a cellulose with acetic acid or acetic anhydride in a solvent in the presence of a catalyst to give a cellulose acetate, and subjecting the cellulose acetate to ripening or aging (hydrolysis) in the presence of water in an amount of 10 percent by mole or less. These documents describe that the intermolecular or intramolecular distribution of degree of acetyl substitution can be controlled by the above procedures even if a large amount of catalytic sulfuric acid is used, and that the resulting cellulose acetates have a degree of acetyl substitution at the 2-position of 2DS, a degree of acetyl substitution at the 3-position of 3DS, and a degree of acetyl substitution at the 6-position of 6DS, in which 2DS, 3DS, and 6DS satisfy the following conditions:2DS+3DS>1.80,3DS<2DS, and6DS>0.80
These cellulose acetates each have a uniform distribution of the degree of acetyl substitution, where acetyl groups are uniformly distributed, and have high solubility in a solvent even when they are cellulose triacetates having a high degree of substitution of, for example, from 2.636 to 2.958. These cellulose acetates, however, have little margin of introducing other acyl groups than acetyl group into the 2- and 3-positions, because they have a high degree of acetyl substitution at the 2- and 3-positions.